1. Field of the Invention
The present invention relates to a technique for diagnosing an abnormality in a selective catalytic redaction catalyst (an SCR catalyst) disposed in an exhaust passage of an internal combustion engine.
2. Description of the Related Art
A conventional exhaust gas purification apparatus of an internal combustion engine includes an SCR catalyst and an addition device that adds an additive constituted by ammonia (NH3) or an NH3 precursor to exhaust gas flowing into the SCR catalyst. In a conventional technique for detecting an abnormality in this type of exhaust gas purification apparatus, an abnormality in the exhaust gas purification apparatus is diagnosed using an amount of NOx (referred to hereafter as a “NOx inflow amount”) flowing into the SCR catalyst as a parameter. For example, a method of calculating a NOx purification ratio (a ratio of an amount of NOx purified by the SCR catalyst to the NOx inflow amount) of the SCR catalyst using the NOx inflow amount as a parameter, and diagnosing an abnormality in the exhaust gas purification apparatus on the basis of the calculation result may be used (see Patent Literature 1, for example).